This invention relates to a placer/spreader for receiving mixed concrete or base materials and distributing these materials in windrows on subgrade in the path of a slipform paver. More particularly, a placer/spreader spanning a road subgrade is provided with a roll in/roll out conveyor for receiving concrete or base material from an access road alongside the road subgrade and thereafter distributing the concrete onto the road subgrade for slipforming by a following paver. The placer/spreader includes a power unit offset from the roll in/roll out conveyor, and a beam reinforced with the conveyor track for strengthening the placer/spreader frame and supporting crawler tracks. With the roll in/roll out conveyor telescoped within and underneath the frame, the placer/spreader frame folds into a compact simplified transport configuration for shipping on a single trailer between job sites with reduced set-up time and overhead.
Slipform pavers are commonly utilized for paving reinforced and non-reinforced concrete roadways and airfield pavements. It is common practice for the concrete delivery trucks to back up on the subgrade and dump the contents of the truck on the subgrade in the path of a slipform paver and thereafter slipform the placed concrete into the final profile of the specified concrete slab section. However, in some cases it is not possible for the trucks to drive on the subgrade of the roadway in the path of the slipform paver and a separate access road must be employed alongside the road.
The placer/spreader here disclosed has utility where an access road is required. Access roads are required along the road subgrade when the subgrade in front of the slipform paver is not suitable for driving and dumping (such as when the subgrade is extremely porous, for example where superior drainage under the placed pavement is desired) or is too soft, thus not being supportive enough for the delivery trucks. It also has utility to receive and distribute concrete over the top of pre-placed dowel basket assemblies or continuous reinforcing bar reinforcement secured to the subgrade that would otherwise block the path of the concrete delivery trucks.
Placer/spreaders of the prior art typically utilize a side delivery conveyor for receiving from concrete delivery trucks traveling along the access road (alongside the road subgrade) and then distributing freshly mixed concrete in front of the slipform paver on the subgrade to be paved. These side receiving conveyors are of two types: hinged conveyors and so-called “roll in/roll out” conveyors.
Hinged conveyors extend to the side of the placer/spreader at the access road for receiving already mixed concrete from a delivery truck and transporting and distributing that concrete onto the subgrade in the path of the slipform paver. Such hinged conveyors are relatively inexpensive and well understood in their operation. Such hinged conveyors do have disadvantages and advantages over roll in/roll out conveyors.
First, hinged conveyors typically hinge down onto and up out of an access roadway at the side of the road surface to be paved each time a delivery truck must pass. There is frequently insufficient room on the access road for trucks to drive around the hinged conveyor when it is in the down position. The access road is only wide enough for one truck to pass. Before a hinging up movement can occur, the conveyors have to be emptied of concrete. Only when the conveyors are emptied of concrete can hinging occur. Thus a precisely timed sequence of truck dumping and conveyor loading, belt emptying, and finally belt hinging up (to allow the next truck to pass) and coordinated delivery truck movement must occur. This precise coordinated movement is not always possible at construction sites, especially where soft road conditions make movement of both the placer/spreader and delivery trucks unpredictable.
Where precise delivery truck and/or placer/spreader movement does not occur, collisions between the receiving end of the conveyor and delivery truck frequently occur. This often results in structural damage to the conveyor, intermittent concrete delivery, and ultimately less than optimum slipform paver movement. Conveyor damage can be catastrophic, bringing the entire road building process to a halt. Further, these collisions, intermittent delivery, and intermittent paver movement can cause uneven pavement surfaces with resultant contract penalties for placement of other than level (smooth) pavement surfaces. Since modern road construction contracts provide premium or bonus payment for smooth roadways and deduction from full payment for uneven pavement surfaces, smooth/level pavement surfaces can significantly impact the road contractor financial results on the project.
Furthermore, the time lost in running the conveyor empty prior to hinging up the conveyor reduces the productive ability of the placer/spreader by reducing the number of loads per hour that the placer/spreader can handle. This reduced productivity may require the use of a second placer/spreader in order to absorb the full output of a high production concrete plant.
Many times access road elevations can vary widely when the access road surface is soft. This can cause problems when the receiving end of the conveyor is too high for the truck to dump into. This leads to delay in the dumping of the truck and adversely affects production.
It is known in the prior art that one advantage of hinged conveying is that the angle of the receiving end of the conveyor can be varied hydraulically on the fly to match the slope or uneven elevation of the access road. The disadvantage of the prior art roll in/roll out conveyor is that it had no ability to adjust the angle of the receiving belt to match the slope or uneven elevation of the access road on the fly. The inability of the conveyor to adjust easily for varying access road slopes and elevations also contributes to lost production.
Roll in/roll out conveyors of the prior art have a concrete receiving end and a slightly elevated concrete discharge end. The concrete receiving end typically telescopes out and is supported in a cantilever fashion overlying the access road. This requires the access road to be well-graded, compacted and level. Already mixed concrete is unloaded onto the cantilevered concrete receiving end of the conveyor. The roll in/roll out conveyor then undertakes two discrete movements.
A first movement is the conventional operation of the conveyor transporting the received concrete from the receiving end of the conveyor to the discharge end of the conveyor. Dependent upon the location of the discharge end of the conveyor, concrete is distributed onto the subgrade to be paved.
A second movement is the so-called telescoping movement of the conveyor. Typically, while the conveyor is running in conventional conveying movement (with concrete still on the belt), the entire conveyor telescopes relative to a supporting U-frame so that its discharge end traverses the subgrade to be paved (e.g. “roll-in”). In such traversing of the subgrade, concrete is still being unloaded off the belt and distributed as the discharge end traverses the subgrades to be paved. As much as a third of a truckload of concrete can still be present on the running belt when the belt is being rolled in.
Additionally, and as a consequence of the second movement, the discharge end of the conveyor distributes the remaining concrete on the belt across the subgrade during its traversing movement. Prior to the transverse movement of the conveyor, concrete can accumulate in the traversing path of the telescoping conveyor overlying the subgrade. The conveyor discharge end when equipped with a strike-off plate can collide with and strike off the upper portion of the accumulated concrete pile, further distributing concrete on the subgrade to be placed. This allows the entire truckload of concrete to be discharged without delay.
In contrast to this, a hinge-up conveyor configuration must receive the entire truckload of concrete before the placer/spreader can move ahead. With large truckloads of concrete, many times the concrete pile under the discharge end of the conveyor gets so high that it prevents remaining concrete on the conveyor from being discharged. The concrete backs up on the conveyor. The only way to resolve this situation is to move the placer/spreader and dumping truck ahead to make room under the discharge end of the conveyor so the conveyor can empty. Only when the conveyor is empty can the conveyor receiving end hinge up to allow the next delivery truck to pass.
Because of these distribution characteristics, roll in/roll out conveyors have superior concrete distribution characteristics over hinged conveyors and are more productive. Furthermore because of the inherent weakness of a hinge conveyor to side loads (namely a truck colliding with it) the roll in/roll out conveyor configuration is superior and more robust in construction. Because the hinged conveyor must be able to hinge more than 90 degrees, it is almost impossible to build a hinge with sufficient strength and structural integrity to prevent damage when a truck hits it. The more robust construction possible with a receiving end of a roll in/roll out conveyor makes it capable of colliding with and even pushing delivery trucks, which hinged conveyors cannot. This is important from a standpoint of minimizing potential down time and increasing the productivity of roll in/roll out and hinged conveyors.
Prior art roll in/roll out conveyors are typically supported on a separate support frame (conveyor module). This modular support frame includes paired bolsters aligned parallel to and arranged on either side of the subgrade to be paved. Paired crossbeams span the subgrade between the bolsters and tie the two bolsters together. The conveyor and its overlying support frame (as a module) are inserted between bolster-supported jacking columns with supporting crawler tracks in front of and a traditional paver-like tractor with a power plant behind the conveyor module. The diesel/hydraulic power unit is centered and on top of the tractor frame and provides power for the entire placer/spreader, including the roll in/roll out conveyor. The tractor unit also includes a removable set of rear jacking columns and supporting crawler tracks.
Unfortunately, roll in/roll out conveyors as presently used and implemented on such support frames and tractor frames with power units have several disadvantages.
First, such roll in/roll out conveyors and their supporting structures require heavy-duty construction. When loaded with already mixed concrete their weight increases considerably. Typically, when the conveyor is rolled, it can be holding up to four yards of concrete weighing approximately 3000 pounds each. Thus the supported roll in/roll out conveyors are a heavy dynamic load, placing high load demands on their supporting frames.
These heavy roll in/roll out conveyors are typically provided with two support points. A first support point is adjacent the access road. This support point adjacent the access road enables the extended conveyor to cantilever out into the access road for receiving ready-mix concrete. The second support point is on a rail spanning the width of the subgrade over the roll in/roll out conveyor. When the conveyor telescopes in, severe loading is placed on the support frame through the second support points on the spanning rail. From the spanning rail, the loading is distributed to the placer/spreader frame.
Second, such placer/spreader frames are utilized to support a hydraulic power plant for powering the entire placer/spreader including the roll in/roll out conveyor. Adding the weight of the roll in/roll out conveyor to the same frame supporting the power plant has thus far necessitated the use of two frames. Specifically, the tractor frame is utilized to support a ground-level concrete spreader (such as an auger spreader) and the hydraulic power unit. A second dedicated conveyor supporting frame is utilized for the support of the loaded telescoping roll in/roll out conveyor.
In the mid-1960s, CMI Corporation (originally Construction Machinery Inc.) of Oklahoma City, Okla., manufactured a placer/spreader known as the PST 400 having the above construction with a roll in/roll out conveyor belt. Since then, another manufacturer copied this machine in its entirety. The machine included a main tractor frame with two side bolsters supporting an underlying concrete spreader auger with strike-off and an attached overlying power unit. The main tractor frame had an attaching rear bolster. Attached to the front of the main tractor frame was a conveyor supporting frame (conveyor module) which had an underlying roll in/roll out conveyor and an attaching front bolster. The entire assembly was supported on four jacking columns with crawler tracks.
This machine had superior function and productivity but was bulky, requiring multiple loads to be transported between job sites.
Specifically, three discrete loads must be transported. Further, both assembly and disassembly of the unit require a crane assist. Regarding the loads, a first load includes the main tractor frame with the power unit and an underlying spreader auger assembly. A second load includes the conveyor supporting frame with an underlying roll in/roll out conveyor. A third and final load includes the disassembled front and rear bolsters, each with jacking column and supporting crawler track. Disassembly and assembly of the placer/spreader was and is very time-consuming and takes days to set up and tear down.
Moreover, the two discrete frames, namely the conveyor support frame and the tractor frame, require re-sectionalization when a width change of the placer spreader is required, say to change the width from a standard 24-foot wide pavement to a 30-foot wide pavement.
It should be understood that spreader augers utilized by the prior art are less than optimum. Typically, and because of the limits of the auger flighting and the mass of the auger, a central support bearing is required to support the auger from the main tractor frame. The supported auger includes opposed auger flights terminating at a central bearing. Further, such augers have a diameter in the range of three feet in order to be large enough to spread concrete rapidly. Finally, and assuming that more concrete is placed on one side of the auger support bearing than on the other side of the auger support bearing, redistributing concrete across the central support bearing of the spreader auger is problematic at best. Spreader augers are very costly to operate because the auger flighting and the bearings wear out rapidly when conveying abrasive concrete.
During the inventors' research and work that led to the development of the following described placer/spreader, considerable resistance was encountered from our prospective customers to the use of roll in/roll out conveyor belts. This resistance was a direct result of the extraordinarily difficult and time-consuming set-up, transport and width change costs of prior art machines utilizing roll in/roll out conveyors. Interestingly enough, even though customers do not like the prior art placer/spreader, they often refer to the advantages of roll in/roll out conveyors. Specifically, we realized after our research that a placer/spreader utilizing a roll in/roll out conveyor (that had some limited ability to adjust the angle of the receiving end of the belt to compensate for sloped or uneven access roads) and that could be transported in a single load would have great utility and value to users.
The reader will realize that the above close analysis of failings of the prior art has been our work product, resulting from considerable research. It will be further understood that we have never seen the comments set forth above serially in the prior art together with the problems generated by such prior art. It is well known that the recognition of problems to be solved can constitute invention. Accordingly, we claim invention in recognizing the problems to be solved as well as setting forth the particular solutions to those problems.